Manufacture of a silicon waveguide structure

ABSTRACT

A process for making a silicon rib wavegide structure is described comporising the following steps: 
     (i) forming a window in a protective layer on the surface of a silicon wafer to expose a part of said surface; 
     (ii) depositing a buffer layer at least over said exposed surface; 
     (iii) carrying out an etch step to etch the buffer layer and silicon outside a protected rib portion thereby to form a silicon rib with the buffer layer on its upper surface; and 
     (iv) forming a layer of cladding at least on side walls of the silicon rib.

FIELD OF THE INVENTION

The present invention relates to manufacture of a silicon waveguidestructure.

BACKGROUND TO THE INVENTION

In integrated optical circuits, a silicon waveguide structure typicallycomprises a rib formed in the upper epitaxial silicon layer of asilicon-on-insulator chip. The rib has a top surface and side walls, andhas trough portions on either side of it. The rib serves to confine anoptical transmission mode for light which is contained in the rib andunder the trough portions.

It is often desirable to modify the basic waveguide structure to performa number of different functions. During these modifications, it isfrequently required to treat the top surface of the rib in a mannerdifferently to that of the side walls.

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a process for making asilicon waveguide structure which permits these modifications to becarried out in an accurately controlled fashion. Therefore it isimportant that however the top and side walls are individuallyprotected, this is done in a well-aligned fashion.

According to one aspect of the invention there is provided a process formaking a silicon rib waveguide structure comprising:

forming a window in a protective layer on the surface of a silicon waferto expose a part of said surface;

depositing a buffer layer at least over said exposed surface;

carrying out an etch step to etch the buffer layer and silicon outside aprotected rib portion thereby to form a silicon rib with the bufferlayer on its upper surface; and

forming a layer of cladding at least on side walls of the silicon rib.

Preferred and optional features of the invention will be apparent fromthe subsidiary claims of the specification.

For a better understanding of the present invention and to show how thesame may be carried into effect, reference will now be made by way ofexample to the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a known rib waveguide formed in asilicon-on-insulator chip;

FIG. 2 is a perspective view of a waveguide structure;

FIGS. 3a, 3b, 3c, 3d, 3e, 3f, and 3g are steps in a process for forminga waveguide structure; and

FIG. 4 is a cross-section through a polariser.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The rib waveguide described herein is based on a silicon-on-insulatorchip. A process for forming this type of chip is described in a paperentitled "Reduced defect density in silicon-on-insulator structuresformed by oxygen implantation in two steps" by J. Morgail et al, AppliedPhysics Letters, 54, page 526, 1989. This describes a process for makingsilicon-on-insulator wafer. The silicon layer of such a wafer is thenincreased, for example by epitaxial growth, to make it suitable forforming the basis of the integrated waveguide structure describedherein. FIG. 1 shows a cross-section of an optical waveguide formed onsuch a chip. The chip comprises a layer of silicon 1 which is separatedfrom the silicon substrate 2 by a layer of silicon dioxide 3. The ribwaveguide 4 is formed in the silicon layer 1. FIG. 1 also shows an oxidecladding 5 formed over the rib waveguide 4. Further details of this formof waveguide are given in a paper entitled "Low loss single mode opticalwaveguides with large cross-section in silicon-on-insulator" by J.Schmidtchen et al in Electronic Letters, 27, page 1486, 1991 and in PCTPatent Specification No. WO95/08787.

This form of waveguide provides a single mode, low loss (typically lessthan 0.2 dB/cm for the wavelength range 1.2 to 1.6 microns) waveguidetypically having dimensions in the order of 3 to 5 microns which can becoupled to optical fibres and which is compatible with other integratedcomponents. This form of waveguide can also be easily fabricated fromconventional silicon-on-insulator wafers (as described in WO95/08787referred to above) and so is relatively inexpensive to manufacture.

FIG. 2 shows a perspective view of such a rib waveguide which has formedon a part of its upper surface a buffer layer 6. The buffer layer 6typically has a length of 3 mm or less or preferably 1 mm or less.

As will become clearer in the following, the buffer layer has a numberof different possible functions, in particular because the processdescribed herein ensures that the edges of the buffer layer 6 areself-aligned with the top of the rib waveguide 4. This allows thecladding layer 5 to protect the side walls of the rib 4 while the bufferlayer 6 protects its top surface. This allows for example themanufacture of a polariser by depositing a light absorbing layer on topof the buffer layer 6 but not on the side walls of the rib 4.

Moreover, if the buffer layer and the cladding layer have different etchcharacteristics, it allows the top surface of the rib 4 to be exposedwhile the side surfaces remain protected, or vice versa to allowselective introduction of dopants either into the top of the waveguideor through its sides. This can be done for example to control therefractive index of portions of the waveguide.

A process for making the rib waveguide of FIG. 2 will now be describedwith reference to FIG. 3.

FIG. 3a shows the upper surface of a silicon-on-insulator chip, and inparticular shows the silicon layer 1. A protective layer of oxide 8having a thickness of about 7000 Å is formed on the top of the siliconlayer 1. Using a mask (not shown), a window 9 is etched through theoxide layer 8 to expose the surface of the silicon layer 1 (FIG. 3b). Athin buffer layer 10 is then deposited. The buffer layer is of nitrideand is deposited using an LPCVD (low pressure chemical vapourdeposition) process. For example, the thickness of the buffer layer maybe around 170 Å (see FIG. 3c).

Then, according to FIG. 3d, a pattern of photoresist 12 is deposited. Ofimportance, a central part 14 of the photoresist defines the area wherethe rib waveguide 4 is to be formed.

Then, an etch step is carried out to etch through the buffer layer 10and into the silicon layer 1 to define the rib 4. This can be done as asingle etch step using a known dry etch process, or as a two step etchprocess. This is illustrated in FIG. 3e. The depth of the waveguide isfor example 1.45 μm.

Then, according to FIG. 3f, the resist pattern 12 is removed to have theeffect of leaving a buffer layer 6 perfectly aligned with the side wallsof the rib 4.

Finally, an oxidation step is performed to form the cladding layer 5.The thickness of the oxide layer forming the cladding layer 5 is about0.35 μm. During the oxidation process, the nitride layer 6 on top of therib 4 effectively inhibits oxidation on the top surface of the rib. Onlya very thin oxide layer of about 40 Å will form. This is denoted 16.

FIG. 4 is a cross-section through a polariser which has been formedusing the rib waveguide structure made by the process described above.After the step illustrated in FIG. 3g, a short oxide etch is performedwhich removes the native oxide layer 16 above the buffer layer 6, butstill retains a significant thickness of the cladding layer 5. Then, alight absorbing layer 7 is deposited on top of the buffer layer 6. Thealignment of this layer 7 is less important because the sides of the ribare protected by the silicon dioxide. The light absorbing layer 7 ispreferably a metallic layer, for example aluminium. The metallic layer 7causes attenuation of the light received from the waveguide. Moredetails concerning operation of a polariser of this type are given inour British Patent Application No. 9718346.1.

Another use of the structure illustrated in FIG. 3g is to allowselective introduction of dopant into the rib waveguide structure. Forexample, if dopants are required to be introduced through the side wallsor in the trough portions 18 on either side of the rib 4, then thecladding layer 5 can be etched away using an oxide etch, while thebuffer layer 6 remains to protect the top surface of the rib 4.Conversely, if dopants are required to be introduced into the topsurface of the rib but not into the side walls or trough portions 18,then a selective etchant can be used to etch away the nitride but leavethe cladding layer 5 intact. Thus, the selective etch characteristics ofthe cladding layer and buffer layer allow a number of different dopingpossibilities.

As an alternative to the use of nitride for the buffer layer, a nativeoxide layer may be used. That is, during the step illustrated in FIG.3c, an oxide growth step is performed to grow a thin oxide layer on theexposed surface of the silicon layer 1 in the window 9. Afterwards, thesteps are the same as already described. It can be seen that the effectof this would be to have, in the structure of FIG. 3g, a slightlythicker oxide layer on the top portion of the rib as opposed to thecladding layer 5 on the side portions and trough portions. Once againhowever this allows for a selective etch characteristic, because for agiven etch time, only a certain thickness of oxide will be removed.Therefore, it is possible to remove, for example, oxide from the sidewalls of the rib without removing all the oxide from the top portion.

If a native oxide layer is used, this may have, for example, a thicknessof about 30 Å.

Therefore, overall the buffer layer may have a thickness in the range20-500 Å and preferably in the range 80-220 Å. When considering the useof the buffer layer in a polariser, the thickness of the layer dependson the refractive index of the buffer layer.

By use of a buffer layer on the top of the rib, a pattern can be etchedinto the top of the rib without affecting the side walls.

What is claimed is:
 1. A process for making a silicon rib waveguidestructure comprising:forming a window in a protective layer on thesurface of a silicon wafer to expose a part of said surface; depositinga buffer layer at least over said exposed surface; carrying out an etchstep to etch the buffer layer and silicon outside a protected ribportion thereby to form a silicon rib with the buffer layer on its uppersurface; and forming a layer of cladding at least on side walls of thesilicon rib.
 2. A process according to claim 1, wherein the buffer layercomprises silicon nitride.
 3. A process according to claim 2, whereinthe buffer layer is deposited using an LPCVD process.
 4. A processaccording to claim 1, wherein the buffer layer is a native oxide.
 5. Aprocess according to claim 1, wherein the thickness of the buffer layeris in the range 20-500 Å and preferably in the range 80-220 Å.
 6. Aprocess according to claim 1, which further comprises the step offorming a light absorbing layer on the buffer layer to define apolariser.
 7. A process according to claim 1, wherein the step offorming a layer of cladding comprises carrying out an oxidation stepsuch that a cladding layer of oxide is formed on the side walls of thesilicon rib.
 8. A process according to claim 1, wherein the claddinglayer and the buffer layer have different etch characteristics.
 9. Aprocess according to claim 1, which further comprises selectivelyremoving either the buffer layer from the top surface of the rib or thecladding layer from at least one of the side walls of the rib, andintroducing dopants into the area from which the buffer layer orcladding layer respectively has been removed.